Surface tension. The trigger value for surface activity has been set to 60 mN/m at 20 ⁰C. This value is in accordance with the cut-off value of 60 mN/m as stated in point A.5 of COUNCIL REGULATION (EC) No 440/2008 of 30 May 2008 laying down test methods pursuant to Regulation (EC) No 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH). In this regulation, it is stated "Considering that distilled water has a surface tension of 72.75 mN/m at 20 °C, substances showing a surface tension lower than 60 mN/m under the conditions of this method should be regarded as being surface-active materials." The method described is based on OECD test guideline 115. (TM III 2011, XX XX 2012)
Surface tension. As mentioned in the beginning, one essential property of surfactant is the formation of colloidal-sized aggregates in solutions when the concentration is above the critical micelle concentration (CMC). The CMC value is an important parameter in characterizing the colloidal behavior of a surfactant because it represents the minimum concentration to achieve the greatest performance in applications involving emulsification, dispersion, solubilization, foaming, or simply surface tension lowering. Experimentally, it is determined by observing the break in the plot of a physico-chemical property of solutions versus the surfactant concentration (or the logarithm of concentration). Surface tension, the minimum amount of work required to create unit area of the interface, is such a physico-chemical property that can be used for measuring CMC. Besides the CMC, surface tension measurements also provide many other parameters of a surfactant including efficiency, effectiveness, and surface area per molecule. Measuring the CMC by surface tension is based on the Xxxxx adsorption equation (or Xxxxx equation).1,10-12,32 The basic form of the Xxxxx equation is: dγ = −∑Γidμi i where dγ = the change in surface tension of the solution (mN/m), Гi = the surface excess concentration of the component per unit area of surface (mmol/m2), and dμi = the change in chemical potential of the component in the solution. For an air/two-component solution system (such as air/aqueous surfactant solution), the surface excess concentration of the solvent is zero. Hence, eq 1.1 is reduced to: dγ = −Γ2dμ 2 (1.2) in which 2 designates the solute (surfactant) dissolved in the bulk phase (water). At equilibrium, the chemical potential of the solute is equal in all phases, so μ2 at the surface can be taken as the value in the bulk phase. The chemical potential of the solute can be related to its concentration in the bulk phase by: dμ 2 = RTd ln a (1.3) where R is the gas constant (8.314 J/mol·K), T is the absolute temperature, and a is the solute activity in the solvent. The solute activity, at low concentration, can be replaced by the molar concentration (c) and eq 1.3 is written as: dμ 2 = RTd ln c (1.4) Then, substituting eq 1.4 into eq 1.2 gives: Γ2 = − 1 dγ RT d ln c (1.5) or Γ2 = − 1 dγ
Surface tension. What is the trigger for the surface activity?
